Solid state imaging devices, e.g., CCD, CMOS, and others, may include a lens or a lens structure to direct incoming light onto a focal plane array of pixels. Each one of the pixels includes a photosensor, for example, a photogate, photoconductor, or photodiode, overlying a substrate for accumulating photo-generated charge in an underlying portion of the substrate. The charge generated by the pixels in the pixel array is then read out and processed to form an image. Often the lens or lens structure is part of a wafer level fabrication and imager module assembly process.
FIG. 1 is a diagram of a lens structure 100 that includes a first convex lens 112 and a second convex lens 114 arranged on opposite sides of a substrate 110 to form a double sided convex lens structure known as “biconvex”. Light rays 120 passing through the lens structure 100 are subject to lateral chromatic aberration. Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light, known as the dispersion of the lens. Since the focal length of a lens is dependent on the refractive index of the lens material, different wavelengths of light will be focused at different positions. Therefore, red 120r, green 120g, and blue 120b components of the light rays 120 are focused at different distances from the lens structure 100, which can result in a blurry image. Chromatic aberration of a lens may manifest as fringes of color around an image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis.
A conventional lens structure 100 having spherical lenses 112, 114 may also produce spherical aberration. Spherical aberration is an image imperfection that occurs due to the increased refraction of light rays 120 when the light rays 120 strike a lens 112, 114 near its edge, in comparison with light rays 120 that strike nearer the center of the lens 112, 114. A positive spherical aberration occurs when peripheral rays are bent too much and a negative spherical aberration occurs when peripheral rays are not bent enough.
What is needed is a system and method by which spherical and chromatic aberrations may be corrected in a lens, including one which can be fabricated at a wafer level.